The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A typical multiple speed transmission uses a combination of a plurality of torque transmitting mechanisms, planetary gear arrangements and fixed interconnections to achieve a plurality of gear ratios. The number and physical arrangement of the planetary gear sets, generally, are dictated by packaging, cost and desired speed ratios.
In order to increase the fuel economy of motor vehicles, it may be desirable to stop the engine during certain circumstances, such as when stopped at a red light or idling. The engine is then turned back on when certain conditions are met, such as a throttle being depressed. This automatic engine stop and restart is referred to as engine stop/start. During an engine stop, the hydraulic control system which controls the transmission may lose pressure. After the engine has been shut down and has remained off for an extended period of time, the hydraulic fluid within the transmission the fluid may drain down from the passages of the hydraulic control system into a transmission sump under the force of gravity. Upon engine restart, the transmission may take an appreciable amount of time to establish pressure before full transmission operation may resume.
In some powertrain systems it is generally desirable to have quick shift timing and system recovery. As such, it may be desirable to maintain pressure in a clutch, even when line pressure within the hydraulic control system and/or clutch feed pressure is zero or near zero. In other situations, however, it may be desirable to eliminate clutch feed pressure to a particular clutch, such as when putting a vehicle in reverse. Accordingly, there is a need for a clutch pressure control system that allows for timely shifting and recovery, even if the engine or motor is shut off, but that also allows a vehicle to move in reverse or to otherwise deactivate a clutch or multiple clutches.